1. Field
The present invention relates to bicycle frames constructed of composite materials, and to methods for making such frames. More particularly, the invention relates to generally hollow, single and tandem seat bicycle frames made from composite fibers impregnated with synthetic resins, and to methods for fabricating said frames.
2. State of the Art
Historically, two rider bicycles (tandem) have been constructed of variously sized metal tubes welded together to a desired frame geometry. This geometry is such as to fit the two riders' leg and torso lengths. The tandem frames currently produced require between 10 to 14 individual metal tubes arranged in a basic triangular truss pattern. Design of this conventional, triangular geometric tandem frame to fit particular pairs of riders requires individual selection of all of the triangular segment angles and tube lengths. Further, the resulting frames are usually quite heavy, to provide adequate axial and lateral stiffness to support the two riders, resulting in inefficient performance.
Single rider frame designs are inherently more simple, and constitute most current composite designs. In recent years, appropriately sized composite plastic tubes have been proposed to directly replace the metal tubes of conventional single rider bicycle frames. These composite frames include integral tube joints in place of the previously used welded metal joints, along with similarly constructed supports for the front fork and handlebar assemblies, the seat and the pedal and sprocket assembly. Branched rear wheel support stays are also of composite materials. These frames designs have continued to utilize conventional structural design concepts, replacing all the tubular metal members with composite plastic members designed to provide equivalent structural behavior. The frame structure disclosed in U.S. Pat. No. 4,900,048 perhaps best exemplifies this approach to design of a composite plastic, single seat bicycle frame. Other composite material design approaches include monocoque type frames with a unitary stress skin with internal stiffening ribs, disclosed in U.S. Pat. No. 4,513,986. In some designs, such monocoque frames have provided an outer skin held in place for strength by an innerfilling of foam, such as disclosed in U.S. Pat. No. 3,833,242.
These prior approaches to composite bicycle frames of fibrous materials and synthetic resin are capable of providing sufficient strength and rigidity to the single seat frames. However, they tend to be overly complex, requiring many tubes of differing shapes, and are also unnecessarily expensive because of multiple mold costs. This results from the substitution of anisotropic material into designs which have evolved from the use of isotropic metal tubes. The resulting frame complexity is felt in corresponding complexity and cost of associated exterior molds and the internal pressurizing bladders used to finally shape and cure the hollow frame components into final form.
A departure from the above approach for composite single seat bicycle frame design is found in U.S. Pat. No. 4,923,203, disclosing a design utilizing a single main tube downsloping to span from the head tube to the rear wheel hub, with a cross tube vertically intersecting the main tube to provide a seat support and a pedal support. This design has been designated the "X-frame" because of its shape. This patent also discloses the single tandem composite frame design known to applicant, and it is called the "X-X-frame". However, the disclosed composite structure designs for these frames, and the required method of fabrication, are unnecessarily complicated and expensive, whether for single or dual seat applications. Initially, uncured composite shells for the main tube and the cross tubes are constructed. Subsequently, flaps cut from the bottom and top of the main tube are bent to join sides of the upstanding and downstanding cross tube walls. The junction so formed must then be overwrapped by additional composite layers to provide a unified hollow construction. The uncured frame is then molded in an expensive three-part split female mold using internal pressure provided by a system of internal inflatable bladders.
Clearly, improved composite plastic fiber bicycle frame designs, and methods of fabrication therefor, remain critically needed, especially for dual seat frame applications, that are both structurally satisfactory and economically attractive.